The invention relates to a computed tomography apparatus which includes a radiation source that emits a cone beam and moves along a helical path relative to an examination zone or an object present therein. The invention also relates to a detector unit that is suitable for use in such a computed tomography apparatus.
A computed tomography apparatus of this kind which is known from U.S. application Ser. No. 09/380,972 (PHQ 98.020) includes a detector unit which is connected to the radiation source and is struck by the cone beam after its passage through the examination zone, its shape being such that its outer edges that are mutually offset in the axial direction cover two neighboring turns of the helix. It can be demonstrated that the radiation source then projects the voxels in the examination zone onto the detector unit from an angular range of exactly 180xc2x0. A CT image of suitable quality can be reconstructed from the CT data (CT=Computed Tomography) thus acquired.
An even better image quality can be achieved by means of a computed tomography apparatus that is known from U.S. application Ser. No. 09/368,850 (PHD 98-086) in which the dimensions of the detector unit in the axial direction are such that the projection of the outer edges of the detector unit onto the helix encloses a path of (2n+1)p, where p corresponds to the axial offset of two neighboring turns of the helix. Each voxel in the examination zone is then projected onto the detector unit from an angular range of (2n+1).180xc2x0. As a result, the signal-to-noise ratio in the voxels is more uniformly distributed across the examination zone in comparison with the first mentioned computed tomography apparatus. Moreover, for the same relative speed between the radiation source and the examination zone the signal-to-noise ratio may be a factor of 2n+1 better than in the known computed tomography apparatus; however, to this end the dimensions of the detector unit must be a factor of 2n+1 larger in the axial direction.
However, the dimensions of given detector units cannot be increased at random in the axial direction. For example, when the detector unit is provided with semiconductor chips with detector elements arranged in the form of a matrix, the semiconductor chips must be concatenated in an arc-like manner in a plane perpendicular to the axis of rotation. Because electronic circuitry for processing the signals of the detector elements must be present on one of the four sides of a chip, only two chips can be arranged so as to be directly adjacent in the direction perpendicular to said plane, that is, in the axial direction, the electronic circuitry connected to the individual chips being situated each time on the sides of the chips that are remote from one another. However, because the dimensions of the chips are limited for production-technical reasons, the dimensions of the detector unit are also limited in the axial direction.
It is an object of the present invention to provide a computed tomography apparatus whose detector unit delivers, despite said limitations, CT data wherefrom CT images having an improved signal-to-noise ratio and a more attractive spatial distribution of the signal-to-noise ratio can be reconstructed.
This object is achieved in accordance with the invention by means of a computed tomography apparatus which includes
a scanning unit which includes a radiation source and a detector unit which is connected thereto in order to detect a conical radiation beam, emitted by the radiation source, after its passage through an examination zone or an object present therein,
a drive device for realizing a relative motion in the form of a helix, comprising a rotation about an axis of rotation and a displacement parallel to the axis of rotation, between the scanning unit and the examination zone or the object,
the projection of the edges of the detector unit, being mutually offset in the axial direction, onto the helix including a path of (2n+1)p, where n is an integer xe2x89xa71 and p corresponds to the axial offset of two neighboring turns of the helix, and
the detector unit including a plurality of detector segments that are spatially separated from one another and are mutually offset in the axial direction, each of said detector segments being arranged and shaped in such a manner that its projection onto the helix covers at least two neighboring turns of the helix.
The invention utilizes symmetries in the data acquisition by means of the known computed tomography apparatus. When the projections of the outer edges of the detector unit that are mutually offset in the axial direction in such a computed tomography apparatus cover, for example, five times the distance between two neighboring turns of the helix, such a detector unit can imaginary be subdivided into five detector segments whose projection interconnects each time two neighboring turns of the helix.
The radiation source projects the voxels in the examination zone onto the central detector segment from an angular range of 180xc2x0. This data suffices to ensure complete reconstruction. On the other hand, reconstruction (with an enhanced image quality) is also possible on the basis of the CT data of the three inner detector segments onto which the voxels of the examination zone are projected from an angular range of 3 times 180xc2x0. Finally, reconstruction is also possible on the basis of the CT data of all five detector segments (onto which the voxels of the examination zone are projected from an angular range of 5 times 180xc2x0).
Because the reconstruction process used to process the CT data so as to form a (three-dimensional) CT image is linear, a respective complete CT image can be reconstructed each time from the CT data of the outer detector segments, the central detector segment and the detector segments that are situated therebetween. The invention utilizes this fact to omit individual detector segments or pairs of detector segments (for example, the two detector segments situated between the central detector segment and an outer detector segment). A CT image having a more favorable signal-to-noise ratio and a more uniform spatial distribution of this ratio in comparison with a CT image reconstructed exclusively from the CT data of the central detector segment can then still be reconstructed from the CT data of the detector segments then remaining.
The omission of these detector segments results in vacant areas between individual detector segments which can accommodate, for example the electronic circuitry for the individual detector segments or fixing points for the detector segments or anti-scatter grids arranged in front thereof.
Claim 2 discloses the simplest detector unit possible. However, such a detector unit gives rise to problems when a circular relative motion instead of a helical relative motion takes place between the examination zone and the radiation source. In that case the part of the examination zone that is situated in the plane between the two detector segments cannot be reconstructed without artefacts.
The embodiment of the invention that is disclosed in claim 3 is more attractive in this respect, be it that it requires at least three detector segments. A central slice that is free from said artefacts can be reconstructed, even in the case of a circular relative motion, by means of the detector segment which is situated in the plane of the radiation source.
Claim 4 discloses a preferred embodiment. The radiation load is thus reduced in a computed tomography apparatus used for medical applications.
The embodiment in accordance with claim 5 is suitable for the reconstruction of CT images for several energy ranges, the CT data required for this purpose being simultaneously acquired.
Claim 6 discloses a detector unit in accordance with the invention for a computed tomography apparatus.